1. Field of the Invention
The present invention relates to a control system for an engine equipped with an automatic transmission.
2. Description of Related Art
Typically, an automobile engine has an output control means, such as a throttle valve, which may be mechanical and which is controlled by an accelerator pedal through a mechanical linkage. Since such mechanical throttle valves change their openings in almost fixed or proportional relations with respect to displacement of accelerator pedals, they disable engines with mechanical throttle valves from changing their outputs according to engine operating conditions. On the other hand, a mechanical throttle valve inherently provides a large change in the quantity of intake air admitted into an engine during a small displacement of the accelerator pedal but quite a small change in this quantity of intake air during a large displacement of the accelerator pedal. Accordingly, displacing the accelerator pedal beyond a certain, fairly large extent causes no effective change in the quantity of intake air admitted into the engine, and does not provide a favorable increase in engine output.
To provide a greater flexibility in engine output control, there has been proposed an electrically controlled or motor operated throttle valve device (which is hereafter referred to as an electric throttle valve for simplicity), in which an electric actuator is driven by an electric signal representative of the displacement of the accelerator pedal so as to control the quantity of intake air admitted into the engine. Such an electric throttle valve is able to flexibly change its opening relative to the displacement of the accelerator pedal by electrically processing the electric signal. With electric throttle valves, the flexibility of output control of engines has been improved. One such electric throttle valve is known from, for instance, Japanese Unexamined Patent Publication No. 2-221,658.
In order to improve output characteristics of a power plant, including an engine and an automatic transmission, an electric throttle valve of this kind is designed and adapted to operate in different throttle opening control patterns according to shifted gears of the automatic transmission. To provide favorable power plant output and a decreased difference in power plant output before and after gear shifting, such an electric throttle valve is designed so that its opening changes to a larger degree for higher gears, in which drive power of the power plant is lowered, than for lower gears. As will become clear, this is shown in FIG. 3. However, changing from one throttle opening control pattern to another upon an up-shift operation of the automatic transmission causes a rapid increase in throttle opening. This is accompanied by a rapid increase in drive power of the power plant, and results in what is known as "shift shock."
When the throttle opening control pattern is changed upon a down-shift operation of the automatic transmission, vehicle acceleration plummets at the beginning of the down-shift operation. This is known as "acceleration slump." When an accelerator pedal is depressed for acceleration and a down-shift operation of the automatic transmission takes place, the gear mechanism of the automatic transmission is temporarily brought into an idling condition. This decreases an engine torque transmitted to driving wheels from the power plant, and leads to a deterioration in drive feeling or shift quality during a down-shift.